Forcing people to follow a space or to set this as a default mode (as it appears to be regarding groups), I wonder if it's that people get noticed in their Jive What matters about changes in Subspaces when following the main space.

I just tested this on our hosted Jive 5 UAT pilot environment and here's what I found. (I.e., this is based on just enough Jive experience to be dangerous. I hope a real expert adds to this discussion.)

Short answer: No. When you are following a space, you are not automatically following sub-spaces

When you look at those sub-spaces, the action button at top right will say "+Follow" and not "Following"

Content posted to those sub-spaces will not appear in your "Followed" activity stream

With exception of content that you have authored

When you author anything in a subspace, then that content does show up in "Followed" activity stream

Replies, comments, etc., in response to content you authored in a space that you are not following do not show up in "Followed" activity stream

It's entirely possible that this is configurable and what I am observing is just the way our environment has been set up.